The invention relates to a device for coating, by means of gas diffusion, hollow workpieces made of heat-resistant alloys, such as Ni, Co or Fe base alloys, whose outer and inner surfaces are connected with one another by bores. The device comprises a container which has at least one gas supply line and one gas removal line. The gas removal line is connected behind the inner surfaces of the workpiece to be coated, and has a donor metal in the form of a donor metal body which completely surrounds the outer and inner surfaces of the workpiece to be coated while maintaining a gap.
From U.S. Pat. No. 2,910,382, a process is known for producing surface alloys on metal components. For this purpose, a donor metal in the form of a sintered body is placed at a distance opposite the component surfaces to be coated and, while the inner surfaces are also coated, the hollow spaces of the component are filled with sintered bodies made of donor metal so that a sintered body is situated also opposite the component surfaces in the hollow spaces.
This known process has the disadvantage that inner surfaces which are inaccessible or have a complex design cannot be coated because it is not possible to introduce a donor metal body into the pertaining hollow spaces of the component.
From the European Patent Document EP 0349420, a device is known for the simultaneous coating by gas diffusion of outer and inner surfaces. In the case of this device, the components to be coated are arranged in the upper area of a box. In the lower area, the box has a carrier gas supply line and a carrier grid for receiving an activator powder and donor metal granulates. This device has the disadvantage that forming heavy donor metal gases must rise from the donor metal granulates to the components to be coated, in which case, corresponding to the barometric height formula, a dilution of the donor metal gases occurs in the vertical direction which disadvantageously results in differences in layer thicknesses on the components as a function of their geodetic height with respect to the donor metal. In addition, the device has the effect that the inner surfaces of the component, with an increasing distance of connecting bores between outer and inner surfaces, have smaller coating thicknesses until the coating in the component interior is completely absent.
It is an object of the present invention to provide a device for coating by gas diffusion by which a uniform coating of a hollow component on the outer and inner surfaces is ensured, particularly in the case of long, narrow and inaccessible hollow spaces.
This object is achieved by a device according to the present invention which has the above-described characteristics and in the case of which workpiece holders are arranged in the container. The container holds the workpieces at a geodetically low height with respect to a donor metal. The hollow spaces are free of donor metal, and the gas removal line is constructed as an overflow device or as a front-connected siphon whose overflow level is positioned at the height of the uppermost surface of the workpiece to be coated.
This device has the advantage that, in the interior of the component as well as on the outside, coating thicknesses are obtained which have low thickness fluctuations. By means of the complete surrounding of the component with donor metal, while a gap to the donor is maintained, the surface of the component is supplied with a constantly high concentration of donor metal gas without any diluting effect of the donor metal gas as a result of its high carrier gas proportion, and without donor metal particles in the interior of the component so that the danger of a clogging of the hollow spaces and of the bores between the outer and inner surfaces is avoided.
Using the workpiece holders, which hold the workpieces at a geodetically low height with respect to the donor metal, the components are advantageously held in an inexhaustible high-concentration donor metal gas source. The gas source forms as a result of the small gap to the donor metal body and the surrounding of the components by the donor metal body in the device according to the present invention.
By way of openings, preferably bores, on the outer surface of the component, the donor metal gas can diffuse into the interior and fill the hollow spaces. The filling and supplying of the hollow spaces with donor metal gas may be accelerated by the introduction of gases by way of the gas supply line and the removal may be accelerated by way of the gas removal line connected behind the inner surfaces. This is particularly advantageous in the case of components with very partitioned hollow spaces, relatively small cross-sectional surfaces of the openings in the outer surface in comparison to the inner surfaces to be coated, or in the case of long, small connecting bores between outer and inner surfaces. This device is preferably suitable for the coating by gas diffusion of power unit blades with cooling ducts and cooling-air bores.
The construction of the gas removal line as an overflow device or front-connected siphon whose overflow level is positioned at least at the level of the uppermost surface of the workpiece to be coated has the advantage that the height of the donor metal gas sump can be adapted to the height of the component. Since the heavy donor metal gas can escape only by way of the overflow device as the threshold, an overflow device or siphon ensures a uniformly high donor metal concentration inside and outside the component.
The workpiece holder preferably comprises a conical seat which has a centrally arranged gas removal duct and corresponds with connecting bores leading to hollow spaces of the workpieces. This conical seat has the advantage that not only the workpiece is held in position, but transition pieces for receiving the components can also be used which permit a fast mounting of the components to be coated. For the protection of the surfaces of transition pieces and of component areas that are not to be coated, these surfaces may be coated with a layer of, for example, ceramic slip and may be dried, in which case this layer may be protected from chipping-off or crumbling-away by an embedding in donor metal powders.
An embedding of the workpiece holder with donor metal powder in the bottom area of the device has the advantage that an almost inexhaustible reservoir with a large donor metal surface is available for the donor metal gas supply of the donor metal gas sump. Therefore, a fine-grained powder pouring of donor metal is preferably arranged below the donor metal body.
The donor metal body preferably comprises one or several large-meshed baskets for granulates which are filled with large-grained donor metal granulates. Although large-grained donor metal granulates have a reduced surface on which donor metal gas may form, it can advantageously be layered laterally and above the component in the provided baskets without falling through the meshes of the basket and thus contaminating the component surface or clogging the bores in the outer surface.
If the donor metal body consists of one or several porous sintered bodies, the surface can advantageously be enlarged with a correspondingly higher porosity and baskets for maintaining the gap will not be required because of the dimensional stability of such a donor metal body. Donor metal bodies consisting of one or several porous sintered bodies are preferably used when large piece numbers are to be coated.
Another advantageous construction is a donor metal body which consists of a compact metal into which labyrinth structures are machined for the guiding of the gas. The labyrinth structures advantageously enlarge the donor metal body surface so that a slowly flowing carrier gas can be enriched with donor metal gas before it reaches the surfaces to be coated. This donor metal body, which may also have several parts, is placed in the inverted position over the component to be coated while maintaining a gap of a width of from 0.5 to 50 mm.